1. Field of the Invention
The present invention relates to free-cutting copper alloys, such as those used in all kinds of industries, but especially to alloys used in the field of providing potable water for human consumption.
2. Related Art
Among the copper alloys with a good machinability are bronze alloys such as those having the JIS designation H5111 BC6 and brass alloys such as those having the JIS designations H3250-C3604 and C3771. These alloys are enhanced in machinability with the addition of 1.0 to 6.0 percent, by weight, of lead so as to give industrially satisfactory results as easy-to-work copper alloys. Because of their excellent machinability, those lead-containing copper alloys have been an important basic material for a variety of articles such as city water faucets and water supply/drainage metal fittings and valves.
In those conventional free-cutting copper alloys, lead does not form a solid solution in the matrix but disperses in granular form, thereby improving the machinability of those alloys. To produce the desired results, lead has, heretofore, had to be added in as much as 2.0 or more percent by weight. If the addition of lead in such alloys is less than 1.0 percent by weight, chippings will be spiral in form, such as shown in FIG. 1G. Spiral chippings cause various troubles such as, for example, tangling with the cutting tool. If, on the other hand, the content of lead is 1.0 or more percent by weight and not larger than 2.0 percent by weight, the cut surface will be rough, though that will produce some results such as reduction of cutting resistance. It is usual, therefore, that lead is added to an extent of not less than 2.0 percent by weight. Some expanded copper alloys in which a high degree of cutting property is required are mixed with some 3.0 or more percent by weight of lead. Further, some bronze castings have a lead content of as much as some 5.0 percent, by weight. The alloy having the JIS designation H 5111 BC6, for example, contains some 5.0 percent by weight of lead.
In alloys containing a few percent lead, fine lead particles are dispersed in the metal structure. During the cutting process, stress can be concentrated on these fine, soft lead particles. Consequently, the chips produced when cutting are smaller and the cutting force is lower. Lead particles act as a chip-breaker under these circumstances.
Meanwhile, when 2.0 to 4.5% Si is added to Cu—Zn alloys under a given composition range and production conditions, there appears in the metal structure one or more of Si-rich κ, γ, μ, or β phases apart from the alpha phase. Among these phases, κ, γ, and μ are hard and have totally different properties from Pb. However, when being cut, stress concentrates on the area where these three phases are present so these phases also act as chip-breakers, thereby lowering the cutting force required. This means that although Pb and κ, γ, and μ phases generated in a Cu—Zn—Si alloy have little or nothing in common in their properties and/or characteristics, they all break chips, and as a result, reduce the required cutting force.
Even so, improved machinability of Cu—Zn—Si alloys having κ, γ, and μ phases is not sufficient enough, in some respects, as compared to C83600 (Leaded Red Brass), C36000 (Free-Cutting Brass), and C37700 (Forging Brass) which contain 5%, 3%, and 2% lead, by weight, respectively.
The application of lead-mixed alloys has been greatly limited in recent years, because lead contained therein is harmful to humans as an environmental pollutant. That is, the lead-containing alloys pose a threat to human health and environmental hygiene because lead finds its way into metallic vapor that is generated in the steps of processing such alloys at high temperatures, such as during melting and casting. There is also a danger that lead contained in the water system metal fittings, valves, and so on made of those alloys will dissolve out into drinking water.
For these reasons, the United States and other advanced nations have been moving in recent years to tighten the standards for lead-containing copper alloys to drastically limit the permissible level of lead in copper alloys. In Japan, too, the use of lead-containing alloys has been increasingly restricted, and there has been a growing call for the development of free-cutting copper alloys with a low lead content. Needless to say, it is desirable to reduce lead content as much as possible.
Recent advances have reduced lead content in free-cutting copper alloys to as low as 0.02%, for example, as described in US 2002-0159912 A1 (publication of U.S. application Ser. No. 10/287,921). However, in view of strong public concerns over lead content, it is desirable to reduce lead content even further. Although lead-free alloys are known in the art, for example, as described in U.S. Pat. No. 6,413,330, the present inventor has found that certain advantages exist in having small amounts of lead in the alloy.